This invention relates to a pulse-width multiplier in which an input voltage corresponding to the first factor of a product can be switched, with alternating sign, to a smoothing stage, by means of a switch and an inverter. The switch is actuated by width-modulated pulses representing the second factor of the product.
A multiplier of this type, for forming an output U.sub.a =(x/y).multidot.z from the input voltages x, y, z is commercially available (Time-Division-Multiplier Type EL 1/299.02 of Siemens AG, Price List RE 1, May, 1972, pages 6/33 to 36). In this unit, the input voltage z is applied to the input of a smoothing stage via a first double-throw switch, alternating between directly coupled (at switching time t.sub.1) and, via an inverter, with reversed sign (at switching time t.sub.2). The switch is actuated by means of pulses which are formed from the input voltage x and y, in such a manner that (t.sub.1 -t.sub.2)/(t.sub.1 +t.sub.2)=x/y, so that an output voltage U.sub.a =z.multidot.x/y is present at the output of the smoothing stage (by principle of pulse-width multiplication). A sawtooth generator, a limit indicator (two-level device), and a second double-throw switch are provided for forming the switching pulses. The second double-throw switch passes the input voltage y either directly or, via a further inverter, in alternation, and is switched simultaneously with the switching pulses for the first double-throw switch. To generate the sawtooth waveforms employed, the input voltage x and the output signal y' of the second double-throw switch are fed to the inverter input of an I-stage (integrating amplifier). The output voltage y'=.+-.y of the double-throw switch is also applied to the limit indicator as a limit, and the output signal of the limit indicator operates the second double-throw switch as soon as the auxiliary voltage furnished by the I-stage reaches the limit y. After the second double-throw switch is switched into the position y'=+y, the curve of the auxiliary voltage therefore increases linearly through integration of the input variables (x+y) until it reaches the limit +y after the time t.sub.1. Then the double-throw switch is switched to the position y'=-y and the auxiliary voltage curve decreases linearly during the switching time t.sub.2, corresponding to the input variables (x-y) of the I-stage. For the switching time t.sub.1 and t.sub.2 of the two double-throw switches, the condition x/y=(t.sub.1 -t.sub.2)/(t.sub.1 +t.sub.2) is thereby achieved. This known multiplier contains 6 potentiometers for balancing and normalizing.
Such pulse-width multipliers are distinguished by great accuracy. However, as soon as several such multipliers are required for certain applications, for instance, in controls, the expenditures for switching elements and adjusting labor increase. Furthermore, such pulse-width multipliers are intended for multiplying constant input quantities. With variable inputs, distortions in time can occur through the smoothing stage. This happens, for instance, if machines must be controlled as a function of the positions of rotating machine parts, and if the inputs, variable in time, corresponding to the machine positions, must be multiplied and added together for the analog control, where the individual factors of the products to be added are time-dependent in common. If such a pulse-width multiplier were used for forming each product, superpositions and beats would occur through the addition of the output signals of the individual smoothing stages; these can be avoided only when the time relationships of the individual smoothing stages are carefully matched to each other. However, since only discrete capacity values with considerable manufacturing tolerances are available for the capacitors required for the smoothing stages, conventional pulse-width multipliers are not suitable for such purposes. Thus, in German Pat. No. 19 42 312, for instance, characteristic-multipliers are used for control of an asynchronous machine.
It is an object of the present invention to describe an analog computing circuit for calculating a sum ##EQU2## of products of the input voltages b.sub.i and c.sub.i, which requires a minimum amount of components and adjusting labor, which is designed for processing quantities which vary in time, and which operates according to the principle of pulse-width multiplication.